Integrated Model for Anti-Motoring and Abnormal Frequency Protections in Large Generators

8/20/2019 Integrated Model for Anti-Motoring and Abnormal Frequency Protections in Large Generators

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Integrated Model for Anti-Motoring and Abnormal Frequency Protections in Large Generators

| IJMER | ISSN: 2249–6645 | www.ijmer.com | Vol. 5 | Iss. 7 | July 2015 | 2 |

1.2 Abnormal Frequency OperationOperation at other than normal frequency occurs as a consequence of unfavourable/ faulty conditions

that exist within the power system to which the unit is synchronized and is attributable largely to the mismatches

between generation and power demand. Over frequency operation of a generator occurs when generation far

exceeds load demand. Over frequency can occur after the loss of a large load block or a major power exportingtie lines. On the contrary, under frequency operation can occur as a consequence of insufficient generation due

to the failure of a large generating unit or loss of a major power importing tie line[2]

.

II. CONCEPTS OF ANTI-MOTORING PROTECTION Motoring conditions can occur due to operator‟s mistake during synchronization with machine speed

slower than the power system; circuit breaker failure to open during unit shutdown; or due to a mechanical

breakdown[4]

. Both electrical and mechanical anti-motoring protection systems are usually implemented in

majority of the power plants. Electrical anti-motoring protection is implemented by a directional sensing reverse

power relay that senses reversal of the power flowing out of the generator terminals. Motoring power

requirements at of any generation unit is a function of windage and frictional losses of the prime mover and rotor

in association with the power losses in the generator. The anti-motoring protection operates to isolate the

generator and prime mover from the power system [3]. Typical motoring power requirements for different types

of generating units are as follows [4]:

Steam turbines

a) Condensing types

b) Noncondensing type

1%- 3%

3 +%

Hydraulic Turbines

a) Impulse type

b) Reaction type

0.2% - 2+%

+/- 25%

Diesel engines +/- 25%

Gas turbines 50+ %

Various detection means are provided as part of the generator and its control, but supplementary

reverse power protection device/function (ANSI 32) is recommended. Sensitive numerical relays with

sensitivities down to 1 mA would be necessary when a generating unit is operating on part load[5]

. Selection of

CT and PT instrument transformers along with their interconnecting cables need special attention in such cases.

Theoretically, an anti-motoring device, whether monitoring for secondary watts or current, compute power

requirements of the generating unit as:

(1)

(2)

Where E and I refer to line to line quantities and ϴ is the power factor angle.

Fig. 1 Reverse power measurement principle





2.2 Anti-Motoring Trip LogicSequential logic mentioned above can prevent over speeding of the unit. However, failure of this logic

might result in motoring conditions which are to be prevented by dedicated anti-motoring scheme such as a

separate anti-motoring device/function. This backup element is time coordinated with the sequential logic. The

backup element is interlocked only by generator breaker closed feedback (52a contact). Care should be taken to provide sufficient time delay to the backup protection scheme so as to allow the generating unit to prevail during

power system transients. At the same time, this delay shall not be more than the max. permitted motoring time[3]

.

III. CONCEPTS OF ABNORMAL FREQUENCY PROTECTION Under frequency operation of a synchronous generating unit is more of a concern than an over

frequency operation. Appropriate timely governor action can promptly control an over frequency condition by

reduction of power input to the prime mover. The generator load handling capacity is improved by increased

speeds that prevail during over frequency incidents. On the contrary, excessive loading in the power system is

the root cause of an under frequency event which is beyond the control of the generating stations. In an under

frequency incident, generator loading capabilities are reduced by the reduction in ventilation of the unit due to

reduced shaft speed. Under frequency conditions prevail in an over loaded system resulting in all grid connected

generators exceeding their name plate rating. They would need more ventilation, but due to reduction in shaft

speed, the ventilation capability of the generator deteriorates. Automatic Voltage Regulators of excitation

systems resort to field forcing to sustain system voltage which would further over load field and stator circuits of

the generator[3]

.

3.1 Over Frequency ProtectionOver frequency conditions occur when full or partial load shedding is carried out on a generating unit.

Over frequency conditions does not impose any threat to the generating unit and normal governor control action

could bring the unit back to rated synchronous speed without the need to trip the generator. Generators are

designed with inbuilt over speed detectors. Over frequency element is used as additional protection with

separately settable alarm and trip thresholds.

3.2 Under Frequency Protection

Generators may operate with under frequency conditions for a prolonged period as a result ofinsufficient load shedding or due to insufficient generation availability. Either case results in overloading of

connected generating units. Gas and steam based generators are particularly at risk to damage from under

frequency operation[12]

.At reduced frequencies, the prime movers are more vulnerable than the generator

themselves due to probable mechanical resonance in the turbine blade structures. If the shaft speed coincides

with natural frequency of the blades, resonance will occur thereby increasing the vibration levels several fold.

Turbine blade structure can crack on account of cumulative damage accumulated due to excessive vibrations.

Steam turbines are more adversely affected by abnormal frequency operation than the generators they drive.

Operation near mechanical resonance will cause increased blade vibration and cumulative fatigue failure. Hydro

turbines are not affected by under frequency operation, however over frequency which is actually over speeding

is a concern that can result in typically 130 to 150% overspeed owing to huge mass and high kinetic energy of

the water in the water conductor system. Excitation system performance in a self-excited generator deteriorates

with under frequency conditions since underfrequency reduces the exciter output voltage and thereby reducing

the generator terminal voltage.

Primary protection against underfrequency operation and over loading conditions is systematic load

shedding. The under frequency function (device 81U) is used as a backup protection. Load shedding schemes are

designed to trip designated distribution substation feeders to cater to maximum possible overload conditions [7].

Permanent and temporary speed droop characteristics of the turbine governor determines the effective operating

frequency in both overloaded and under loaded conditions of the generator [3]

. Underfrequency load shedding is

used for those situations in which a portion of the interconnected systems becomes islanded and there is an

imbalance between the generation and load. The underfrequency load shedding program is normally provided to

restore the balance in an island with more load than generation. It is essential that generating units “hang on” as

long as possible and do not unnecessarily trip while the underfrequency load shedding is in progress. Therefore,

generation tripping relays must be set to coordinate with the underfrequency load shedding program.

Underfrequency load shedding and generator tripping are normally addressed as a regional criteria [2].





Fig. 4 Example of Coordinated system load shedding in a 60 Hz system

[2]

Backup protection for under frequency is employed as a part of generator protection and is necessary to

rule out situations of under shedding. Backup generator protection in case of inadequate load shedding would be

in the form of multilevel under frequency tripping as shown in Fig.4. A multi-level, time-delayed under

frequency function is preferable for each frequency band on turbine manufacturer‟s limit curve[3] an example of

which is shown in Fig. 5 for a 60 Hz system.

Fig 5. Accumulating timer scheme [3]

IV. MODELING The generated voltage is stepped up to the transmission level by use of a Generator step-up transformer.

For the sake of simplicity of the model, the turbine generator system with governor and generator step up

transformer have been considered as a single entity. All the voltages, currents and frequency monitored and

illustrated refer to the secondary side of the generator transformer high voltage side.

The anti-motoring device function (32) and abnormal frequency functions which are integral part of a

generator multifunction protection relay have been implemented as separate Simulink functional blocks.

Abnormal frequency block detects both over frequency (81O) and under frequency (81U) in one unit and also

employs turbine-governor loop since frequency is controlled by active power which in turn is regulated bygovernor.





Fig. 6 SLD of power system model

4.1 Modelling Of Anti-Motoring Protection Function

The anti-motoring function is implemented by a directional element that receives current and voltage

signals as input. From their phasor relationship, the directional element determines the reversal of power flow.

Directional element feeds the input of a time delay cum threshold decision element which decides by the use of

integrators and threshold value whether a reverse power condition exists for sufficiently long time period. The

output of this stage is fed to trip coil of circuit breaker through a hold logic. In actual power systems, the relaytrip decision is given to a lock out relay (ANSI 86) that act as a holding contact [11].

Fig. 7 Simulink based model with directional elements for detection of motoring conditions

4.2 Modelling Of Abnormal Frequency Protection FunctionProtection function model for abnormal frequency consist of a Frequency measuring unit that

determines the current frequency of the system using the voltage signal input from the voltage transformer.

Fig. 8 Frequency measurement[11]

The frequency measuring block gives forwards its output to the abnormal frequency detection unit

which checks whether the measured frequency is an under frequency condition or an over frequency condition.

In either case, a timer delay achieved by threshold value and integrator waits for the predetermined time duration

after which trip decision is extended to the circuit breaker [12].





Fig. 9 Abnormal frequency function control signal using fuzzy controller

The measured frequency is also fed through a feedback loop to the turbine governor blocks. The output

from the governor block is fed as mechanical input to the synchronous machine via a fuzzy logic controller asshown in Fig.9. Fuzzy Logic is incorporated to control the generator mechanical input in order to cope up with

the variation of load demand and frequency occurring in the power system during the running conditions. Fuzzy

inference systems also referred as fuzzy models are schematically shown in Fig. 10.

They are composed of 5 blocks. A rule- base containing a number of fuzzy if-then rules.

A database which defines the membership functions of the fuzzy sets used in the fuzzy rules

A decision-making unit which performs the inference operations on the rules,

A fuzzification interface which transform the crisp inputs into degrees of match with linguistic values,

A defuzzification interface which transform the fuzzy results of the inference into a crisp output.

Fig. 10 Fuzzy Input Output Model

Fig. 11 Fuzzy model





Above figure shows that two inputs are taken into consideration, one is error and another is change in

error of the frequency difference from the 50Hz reference.

Fig. 12 Rules of fuzzy model

A rule base as shown in Fig. 12 has been implemented using Matlab fuzzy toolbox as listed below: If (E is a Negative) and (CE is negative) then output is Min If (E is a Negative) and (CE is Positive) then output is Zero

If (E is a Positive) and (CE is negative) then output is Zero

If (E is a Positive) and (CE is Positive) then output is Max

Instead of controlling the governor and turbine output using standard PID a fuzzy logic model has been

incorporated to improve the stability of the system.

Fig.13 Integrated MATLAB simulation model used for evaluation of both protective functions.

V.

SIMULATION AND R ESULTS

The generator – transmission network model with anti-motoring and abnormal frequency detection

functions are simulated under four different cases. By varying the mechanical input to the synchronous machine

as a function of time, anti-motoring conditions have been simulated. A three phase power source is made active

during anti-motoring simulations to recreate conditions of other generating resources that may be connected to

the network. Likewise, by modulating the loads on the transmission network as a function of time, various

conditions of under frequency and over frequency have been simulated. Following are the illustration of results

of simulations under four cases.

Case1: Sudden Loss of LoadThis case was to simulate over frequency conditions and check for the operation of the relay. Initially

the generator was supplying a load of 180 MW. After 50 seconds of operation load has been first decreased from

180 MW to 150 MW thereby disturbing the generation- load balance and creating over frequency conditions.





Fig. 14 Response to load loss events (over frequency conditions)

The turbine governor control takes over and generator output is adjusted within 5 seconds. The time

delay function inbuilt into the frequency element resets before it could trip there by establishing the stability of

the over frequency element of the model. Further after 55 seconds, load on the generator has been further

decreased to 80 MW. This large load rejection results in over frequency conditions that could not be

compensated by turbine – governor loop within the permitted time window of 5 seconds provided by abnormal

frequency function. Relay therefore operates and trips the HV breaker through the over frequency element.

Case 2: Sudden Loss Of Mechanical Input While Generator Is Supplying Normal LoadThis case was to simulate reverse power conditions due to mechanical problems in the prime mover. In

this case, the machine was catering to a fixed and continuous load of 110 MW when the mechanical shaft torque

from the turbine has been increased to 145 MW at 15 seconds. The generating unit adapts to the new input after

brief oscillations. After 40 seconds, turbine input was further reduced in a single step to 50 MW. Again the

generating unit is found to adapt to the new turbine input with oscillatory response that swings below 0 MW with

transient excursions on the negative side.

Fig. 15 Response to failure of turbine mechanical input (motoring conditions)





The inbuilt time delay prevents this transient reverse power condition from tripping the unit. Further

after 57 seconds, the turbine input was reduced to -15 MW permanently. This resulted in reversal of power flow

into the generating unit from the connected grid and anti-motoring protection function picks up after the elapse

of time delay at 63 seconds, thereby isolating the generator and its field breaker. It is pertinent to mention that

the action of sequential trip logic of the control system has not been taken into account in the model and the

machine was to be protected only by the backup anti-motoring protection after a fixed time delay.

Case 3: Sudden Excessive Loading On Generator With Insufficient Generation ResourcesThis case was to evaluate the performance of the relay during operation of generator in under frequency

conditions. In this case the load on the generating unit has been increased from 117 MW to 215 MW after initial

operation of 55 seconds. This creates excessive load on the generator that is beyond its capacity. Due to the large

imbalance between generation and load the frequency drops to 45.5Hz. This results in an underfrequency

condition. The system attempts to compensate by raising the output power as is evident from figure below.

However, even after elapse of 5 seconds time at 60 seconds, under frequency conditions continue to

persist and the frequency was still out of range. As a consequence to the above, the underfrequency protection

function picks up at 60 seconds and trips the HV breaker isolating the system from the grid. This action of the

protection system in real time would have further aggravated the under frequency conditions, but was essential to

protect expensive capital equipment such as generating units from damage.

Fig. 16 Response to overloading (under frequency conditions)

Case 4: Cumulative Case To Evaluate Stable Operation With Temporary Load FluctuationsThis case was to evaluate the stability of the generating station to abnormal frequency conditions. In

this case, the load on the generating unit has been increased from the normal constant load of 150 MW to 180

MW at 50 seconds. This action results in dip in frequency. The under frequency function is activated but waits

for the time delay. However, after 4.9 seconds load was reduced to 120 MW load. The underfrequency element

resets before the maximum allowed time delay. However, the over frequency element becomes active and waits

for the time delay. Exactly after 4.9 seconds, the load has been restored back to original 150 MW.





Fig.17 Stability of the model to transient load variations

Though the over frequency function got activated, it was reset as the generation – load imbalance was

restored within the permitted time delay and the over frequency device resets. The responses recorded in the

traces of Fig.17 establishes the stability of the stability of abnormal frequency protection function in the model.

Table I. Summary of Relay Performance Results

Case No.Description of simulated Event

Activated Protectionfunction

Relay trip

decision

1 Load is decreased from 180 MW to 150MW at 50 seconds (normal operation)

After 55 seconds load is further reduced to 80 MW indefinitely Over frequency

Block

Trip

2 Generator was supplying a constant load of 110 MW when the mechanical inputto the turbine was increased to 145 MW after 15 seconds.

After 38 seconds mechanical input to the turbine was decreased to 90 MW.

At 57 seconds, mechanical input from the turbine was reduced (reversed) to -15

MW (motoring) with excitation active. Anti-motoring

Block

Block

Trip

3 Loading on the generator has increased from 117 MW to 215 MW after 55seconds resulting in 45.5 Hz frequency

Under Frequency Trip

4 Load increases from a minimum continuous load of 150 MW to 180 MW after

50 seconds for a duration of 4.9 seconds after which load has been reduced to

120 MW.

After further 4.9 seconds Load is restored back to 150 MW.

Under Frequency

Over Frequency

Block

Block

VI.

CONCLUSION Generation utilities with large generating units usually deploy integrated, multifunction numerical

protection relays for protecting their generators during emergencies. Typical generator multifunction relay can

host several protection functions such as differential, stator earth fault, back up impedance, rotor earth fault etc

included in one compact single package. Amongst all these protection functions two protection functions viz.,

anti-motoring and abnormal frequency protections have been discussed. An integrated model comprising of a

synchronous generator, loads and sources along with both of these functions has been developed. The simulation

results have been illustrated and analyzed to evaluate the performance of these protection functions. The results

prove that the protection functions are found to be performing as intended. Incorporation of other protection

functions such as differential, stator over current etc. of into the integrated model can be taken up as a future

work.





AppendixThe power system model used for simulation and demonstration of the protection functions consist of

the following components

a. Synchronous generating unit – Active Power = 200 MW; rated speed = 166.6 rpm, Frequency, 50 Hz with

shaft connected hydraulic prime mover and inbuilt governor and separately excited, 400V static exciter.

b.

Transmission voltage Vline-line = 132 KV; Suitable step up transformer arrangement (not included in themodel) is presumed to be part of the generating unit.

c. SF6 based High Voltage Circuit Breaker of 1000A and SC rating of 40 KA

d. Network variable load varying from 180 MW to 80 MW.

e. External Generating resource = 200 MW.

R EFERENCES

[1]. ANSI/IEEE Standard C37.102-2006, “IEEE Guide for AC Generator Protection,”

[2]. IEEE Standard C37.106- 2004, “IEEE Guide for Abnormal Frequency Protection of Power Generating Plants,”

[3]. Donald Reimert, Protective Relaying for Power Generation Systems (CRC Press, 2006)

[4]. W. A. Elmore, Protective Relaying Theory and Application, (New York: Marcel Dekker, 2004).

[5]. J. Lewis Blackburn and Thomas J. Domin Protective Relaying – Principles and Applications (third edition, CRC Press)

[6].

Mason C. R. The Art & Science of Protective Relaying (John Wiley & Sons Inc., New York, 1967)

[7].

Kundur P (1994) Power system stability and control (McGraw Hill, New York, NY, USA)[8].

ABB,"SRW Reverse Power Relay” Avaialble on: http://www05.abb.com/global/scot/scot229.nsf

/veritydisplay/5a5acc6d19070dd985256ead0068d04c/$file/41-252A.pdf

[9].

“Generator Protection Using Multifunction Digital Relays,” by BeckWith Electric Co.Inc.

[10].

J. L. Blackburn, Symmetrical Components for Power Systems Engineering (New York: Marcel Dekker, 1993). [11].

M. M. Aman;, et al., " Modeling and Simulation of Reverse Power Relay for Generator Protection," IEEE International

Power Engineering and Optimization Conference (PEOCO2012), Melaka, Malaysia, June 2012

[12].

M. M. Aman;, et al., "Modeling and Simulation of Digital Frequency Relay for Generator Protection," IEEE

International Conference on Power and Energy (PECon), Kota Kinabalu Sabah, Malaysia, June 2012.

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Integrated Model for Anti-Motoring and Abnormal Frequency Protections in Large Generators

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